1. Field of the Invention
Embodiments of the present invention relate generally to disk drives and, more particularly, to a method of writing a preamble field on a disk drive to reduce track squeeze.
2. Description of the Related Art
A disk drive is a data storage device that stores digital data in concentric tracks on the surface of a data storage disk. The data storage disk is a rotatable hard disk with a layer of magnetic material thereon, and data is read from or written to a desired track on the data storage disk using a read/write head that is held proximate to the track while the disk spins about its center at a constant angular velocity.
To properly align the read/write head with a desired track during a read or write operation, disk drives generally use a closed-loop servo system that relies on servo data stored in servo sectors written on the disk surface when the disk drive is manufactured. The servo sectors are written between user data fields on the track of interest. These servo sectors form “servo wedges” or “servo spokes” from the outer to inner diameter of the disk, and are either written on the disk surface by an external device, such as a servo track writer, or by the drive itself using a self servo-writing procedure. The read/write head can be positioned with respect to the data storage disk by using feedback control based on servo information read from the servo wedges with the read element of the read/write head. The servo sectors provide position information about the radial location of the read/write head with respect to the disk surface in the form of servo patterns or “servo bursts.”
During the process of writing the servo wedges on a disk, servo sectors are typically written on the disk one data track at a time. Due to fluctuations in read/write head position while writing the servo patterns for a given data track, the servo patterns for the data track do not form a perfect circle on the disk. Instead, each servo pattern is generally written at a location having a certain displacement, or “runout,” from the ideal track position. This displacement of servo patterns from the ideal track position is referred to as “written-in” repeatable runout (RRO).
As is known in the art, written-in RRO that produces high-frequency disturbances in the position of a read/write head can be readily compensated for during normal operation by implementing correction factors for each servo sector to facilitate smooth and controllable travel of the read/write head along a data track of a storage disk. However, low-frequency fluctuations in head position also generally occur as the servo patterns are written for a specific data track, and therefore produce low-frequency disturbances in the position of the read/write head during normal operation. Such low-frequency disturbances in head position, e.g., fluctuations having a frequency of less than about 500 Hz, are difficult to compensate for and can produce track squeeze, as illustrated below in FIG. 1.
FIG. 1 schematically illustrates a portion of a storage disk 100 and the paths followed by a head 107 as servo wedges are written to a first data track 101 and a second data track 102. Low-frequency fluctuations in the position of head 107 during the process of writing servo wedges 111-115 cause head 107 to vary from ideal track positions 101A, 102A, thereby producing track squeeze between first data track 101 and second data track 102. Track squeeze occurs when track-to-track spacing is inadequate to ensure the data integrity of adjacent data tracks. When writing the servo wedges for first data track 101, head 107 follows a write head path 110 that varies from ideal track position 101A at a low frequency. As a result, the servo information 140 for servo wedges 111-115 for first data track 101 is located along write head path 110, rather than along ideal track position 101A. Similarly, when writing the servo wedges for second data track 102, head 107 follows a write head path 120 that varies from ideal track position 102A at a low frequency, and servo information 150 for servo wedges 111-115 for second data track 102 is located along write head path 120, rather than along ideal track position 102A. Consequently, first data track 101 can vary from ideal track position 101A and second data track 102 can vary from ideal track position 102A in such a way that first data track 101 and second data track 102 partially or completely overlap. As a result, data stored in one track can overwrite data stored in an adjacent track, which is highly undesirable.
Furthermore, the low-frequency track squeeze illustrated in FIG. 1 is known to be the exacerbated by position error produced by preamble phase shift. In hard disk drives using null-pattern demodulation, the preamble field for each servo wedge provides a timing reference for the servo wedge, and preamble phase shift can occur due to the non-ideal shape of the magnetic flux transitions making up the preamble field of servo patterns written to servo wedges 111-115. Ideally, the preamble fields of adjacent data tracks are made up of linear magnetic flux transitions that are connected, or “stitched,” together at a preamble stitch line between the adjacent data tracks to form a radially continuous line from track-to-track. In practice, such magnetic flux transitions are often written with unwanted curvature and/or tilt, producing significant discontinuity at the preamble stitch lines between data tracks. Such discontinuity can affect timing reference accuracy provided by the preamble field when a write head is positioned over the preamble stitch line. Thus, when head 107 is positioned over a preamble stitch line between two data tracks during normal operation, inaccuracy of the timing reference near the preamble stitch line can produce significant additional head position error. Such head position error can exaggerate any track squeeze already present between first data track 101 and second data track 102.
In light of the above, there is a need in the art for a system and method for preventing track squeeze in hard disk drives that use null-pattern demodulation schemes.